The invention relates to a method and to an apparatus for a self-regulating replenishment, from a reservoir, of a helium pool which evaporates, or boils, at subatmospheric pressure to cool the superconductive rotor of an electrical machine, such as a generator, part of the helium changing to the vapor phase during the transfer and both phases being fed into the helium bath at a distance from the axis of rotation.
In order to attain high reliability during the operation of electrical machines employing rotating superconducting field windings, it is necessary to feed the liquid helium in such a manner that operation of the machine will not be interfered with even if there is a malfunction in the cooling system. This "decoupling" of the electrical machine from the cooling system is assured in a simple manner by connecting a reservoir for liquid helium between the machine and the cooling system. The pressure in this reservoir is advisably at the atmospheric level or, in order to prevent the entrance of impurities into the liquid helium from the free atmosphere, at a slightly higher level.
To attain high current densities, the superconducting rotor winding is cooled, for example, with helium which boils, or evaporates, at a reduced pressure of a few tenths of a bar and thus has a boiling temperature of T&lt;4.2.degree. K. This subatmospheric pressure in the rotor can be maintained in a simple manner by appropriately guiding the stream of exhaust gas. Since the exhaust gas leaves the rotor at ambient pressure after it has absorbed heat generated therein, additional pumps are not required in the cold portion of the rotor to produce the subatmospheric pressure.
Essentially three methods are known to solve such a problem. For example, Bejan in "Improved Thermal Design of the Cryogenic Cooling System for a Superconducting Synchronous Generator", Thesis, MIT (1974) and Eckels, in U.S. Pat. No. 4,056,745 propose to expand by means of a choke valve the incoming helium to the subatmospheric pressure existing in the rotor (Joule-Thomson expansion). The valve must be actively controlled corresponding to the flow of helium required in the rotor.
Other proposals, such as in U.S. Pat. Nos. 4,082,967 and 4,048,529 and "Superconducting Generator Design" EPRI-EL-577 (November, 1977), are based on the fact that the helium flowing into the rotor contains a relatively large proportion of vapor due to the thermal losses in the transfer conduit. Therefore, the rotating feeder line is designed so that liquid and vapor are spatially separated, at least in the radial portion of the feeder line. The pressure curve in this conduit is given by the vapor compression due to rotation. This conduit opens into the liquid boiling, or evaporating, at subatmospheric pressure at a point where the hydraulic pressure of the liquid is equal to that in the vapor column. This replenishing system is self-regulating as long as the vapor component does not become too low.
Another method is described in the text "Cryogenics" 17, 429 (1977). In this process, pure liquid is introduced through a radial feeder line and is fed into the helium boiling at subatmospheric pressure at the point where the hydraulic pressures are identical. The radius at which the introduction occurs is greater then when a vapor-liquid mixture is fed in. For example, in a 50-Hz rotor, the radius must be greater than 0.33 m. A further drawback is that only pure liquid is conducted through the radial feeder line.